30 THE HYDROMEDUSAE 



the entocodon and exumbral ectoderm of the gonophore are 

 then developed from the inner layer, the outer layers remaining as 

 cups or strands of blastostylar ectoderm (Fig. 36) between the 

 gonophore and the perisarc of the gonotheca (gubernaculum). 

 Gemmation from a medusoid has rarely been noted (Thaumantias 

 Sars, 36). 



Fission in hydroids has occasionally been observed under a 

 curious form ; a small piece of coenosarc at the end of a growing 

 branch becomes nipped off from the colony, and gives origin to a 

 hydrorhiza, from which a new colony is produced by gemmation 

 (Schizocladium Allman, 1). In medusoids fission is very rare 

 (Eucope Lang, 34 ; Brooks, 35). 



SEXUAL REPRODUCTION. The male and female cells are as a 

 rule borne upon separate colonies, but both may occur in different 

 gonophores on the same colony (Diphasia fallax). Both have even 

 been recognised in the same gonophore, but in this case the male 

 cells alone come to maturity (Gonothyraea). One or more ova may 

 be present in each gonophore ; when the potential ova are 

 numerous, one, or one at a time, generally matures at the expense 

 of the rest. 



The general outline of development indicated above as occurring 

 in the Anthomedusae holds good also for this group. 



ORDER 3. Trachomedusae. 



DEFINITION. Hydromedusae in which the medusoid develops 

 directly from the ovum (hypogenesis) ; no alternation of hydroid 

 and medusoid generation occurs. The chief sense organs are ten- 

 taculocysts, with endodermal otoliths, generally enclosed in vesicles. 

 The generative organs lie on the radial canals. 



The bell of the MEDUSOID in this group (Figs. 38, 39) is generally 

 hemispherical, with a thick mesogloea (Geryonia), but is sometimes 

 thin, and conical or prismatic (Aglantha) ; it is always of firm con- 

 sistence and is provided with a strong velum. The edge of the bell 

 is provided with a special ring of cnidoblasts, with the usual nerve 

 ring, and in some cases with spiral marginal cirrhi, but the marginal 

 tubercles and funnels of the Leptomedusae are not represented. 

 Suckers are rarely developed on the edge of the bell (Pectanthis). 



The primary or perradial tentacles are solid, with a cartilaginous 

 endodermal axis ; between them are often developed interradially 

 secondary tentacles, which are also solid ; both primary and 

 secondary tentacles may be either lost or retained, and replaced or 

 supplemented by tertiary hollow tentacles. The tentacles are 

 tipped by a sucker in a few genera (Pectanthis). 



In a few cases the perradial tentacles are alone developed, 

 either four (Liriope) or six (Geryonia) in number. To these may 



